Telomerase is a ribonucleoprotein enzyme that catalyzes telomere synthesis and other non-telomere lengthening activity. Telomerase activity is tightly regulated during T cell development, differentiation, and activation. With age, telomere shortens in peripheral lymphocytes but it is unknown whether reduction of telomerase activity in lymphocytes also occurs with age. To address this question, we analyzed telomerase activity in T and B cells from 200 BLSA participants with 2 samples separated by an average 5-year of each subject. We found that telomerase activity declined with age in resting and in vitro activated T cells, and that a significant correlation between telomere length and telomerase activity in resting T cells. In addition, we found that telomerase activity declined also in the resting B cells but not in vitro activated B cells. Currently, we tried to uncover the mechanism underlying age-associated decline of telomerase activity in lymphocytes. Telomerase consists of two core components: telomerase reverse transcriptase (TERT) and telomerase RNA template (TERC). TERT serves as a rate-limiting factor whereas TERC is present ubiquitously. TERT mRNA is detectable in resting T cells and rapidly upregulated upon T cell activation. As multiple TERT mRNA alternatively spliced products (ASPs) have been detected, it is unclear whether the mRNAs are full-length (FL) and/or loss-of-function ASPs in T cells. To determine this, we have analyzed the most well characterized hTERT mRNA isoforms FL and 3 types of ASPs (alpha, beta, and alpha+beta) in T cell subsets in relation to resting and activation. Sorted CD4+ and CD8+ nave, central, and effector memory T cells from 12 adults 6 young (&#8804;35 years) and 6 aged (&#8805;70 years) were analyzed by RT-qPCR for TERT isoforms. We found that both FL and ASPs were present in freshly isolated resting and in vitro activated T cells of all six subsets regardless of age. To determine if FL and ASPs were exclusive to individual cells or co-expressed within single cells, we analyzed single cells from 5 donors (32-46 years old). Combined results from six subsets of cells (total 97 single cells) showed exclusive expression of FL TERT or ASP in 65% and 16% of cells analyzed, respectively, while 19% of cells expressed both. Overall, FL TERT could be detected in 84% of single cells. Currently, we are analyzing if there is any association of TERT FL or ASPs with the state of T cells such as resting, activation, proliferation and aging. Currently, we are screening potential inhibitor(s) for TERT splicing to evaluate if these inhibitors can preserve to FL TERT and thus extend lymphocyte replicative lifespan.